A Comparison of Operating Strategies – Chlorine vs UV

ABSTRACTCity of Scott’s Valley owns and operates a 1.5 mgd activated sludge wastewater treatment plant. Currently about 0.3 mgd of the secondary effluent is diverted to the recycled water facility.

Two chlorine disinfection alternatives (both using liquid sodium hypochlorite) and UV disinfection were considered to all the facility to achieve high level (Title 22) disinfection requirements from the recycled water facility. Alternatives were evaluated on the basis of capital cost, operability, O&M costs, maximization of existing facilities, and potential for future expansion. The existing sodium hypochlorite system was left in place for effluent discharged to the receiving stream.

With two disinfection systems on site, the City was able to compare the actual operating cost of both chlorine and UV.

INTRODUCTIONThe City of Scotts Valley owns and operates a 1.5 MGD activated sludge wastewater treatment plant. The treatment process train includes influent bar screens with a washer compactor, influent flow equalization with returned activated sludge, fine bubble aeration, secondary clarification, and chlorine contact. Dewatered sludge is landfilled. Currently about 75% of the plant’s 1.0 MGD flow is diverted to the recycled water facility where it is filtered, disinfected and reused for landscape irrigation at local parks and schools.

In 1996, the City entered into an agreement with the Scotts Valley Water District (SVWD) where the district agreed to fund the construction of a recycled water facility at the City’s wastewater treatment plant. The City owns and operates the recycled water facility and is permitted by the California Regional Water Control Board to produce up to 1.0 MGD of recycled water. Recycled water must meet the State’s Title 22 regulations of not to exceed 2.2 total coliform concentrations and not to exceed a daily average turbidity of 2.0 NTU. The SVWD holds a permit to distribute the recycled water to its end users.

The recycled water facility uses a Tetra denitrification down-flow sand media tertiary filtration system. Denitrification is achieved by adding methanol as a carbon source for the remaining nitrifying bacteria to complete the process of turning nitrate to nitrogen gas. Denitrification is a condition of the City’s permit as a portion of the City’s watershed area flows to the densely populated non-sewered San Lorenzo Valley area.

As the City’s wastewater facility occupies a compact site adjacent to commercial and residential development with little space for expansion, site constraints played a key role in the planning process for the recently constructed recycled water facility. The City evaluated disinfection alternatives, UV vs. chlorine (Sodium Hypochlorite, NaOCL). The evaluation included capital costs, operability, O&M costs, maximization of existing facilities and future expansion.

Chlorine – California Department of Health Services (DOHS) guidelines recommend 5 mg/L residual for 120 minutes for unrestricted irrigation. The existing facility only allowed for 45 minutes at 1.5 mgd. Existing basins could not be modified due to space constraints.

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